The present invention relates to a novel positive-working photoresist composition or, more particularly, to a positive-working photoresist composition capable of giving a patterned resist layer having excellent heat resistance and pattern resolution without the disadvantage of scum formation in the development to be suitable for use in the photolithographic patterning works in the manufacturing process of semiconductor devices in which extreme fineness of patterning is essential.
Photoresist compositions used in the photolithographic patterning works in the manufacturing process of semiconductor devices are classified into positive-working and negative-working photoresist compositions, of which positive-working photoresist compositions compounded with an alkali-soluble novolac resin as a film-forming ingredient and a quinone diazide group-containing compound as a photosensitive ingredient are preferred in the manufacturing process of printed circuit boards and semiconductor devices in respect of the high resolution of patterning as compared with negative-working photoresist compositions.
As a remarkable trend in the semiconductor devices in recent years, the degree of integration in the devices such as ICs, LSIs and the like is increasing year by year so that the fineness of photolithographic patterning must be so fine in the half-micron order to the quarter-micron order. Along with the trend toward extreme fineness of the photolithographic patterning, the major current of the etching process for the pattern-wise etching of the substrate surface with the patterned resist layer as a protective masking layer is the dry etching process undertaken at an elevated temperature so that the patterned resist layer formed from a positive-working photoresist composition is strongly required to be highly resistant against etching or, namely, to have high heat resistance in addition to the requirement for a high resolution in patterning.
Another requirement for a positive-working photoresist composition to comply with the recent trend toward increased fineness in patterning is that the patterned resist layer after the development treatment is absolutely free from occurrence of scum or residue of the resist layer because even a trace amount of scum seriously affects the quality of the patterned resist layer.
In view of the above described requirements for a positive-working photoresist composition, various proposals and attempts have been made heretofore for the improvement thereof. As an approach therefor, improvements have been proposed for the quality of the alkali-soluble novolac resins compounded in the positive-working photoresist compositions. For example, Japanese Patent Kokai 62-172341 proposes a novolac resin for positive-working photoresist compositions which is prepared by the condensation reaction of a cresol mixture and formaldehyde with oxalic acid as the catalyst in the presence of a good solvent for novolac resins such as 2-ethoxyethyl alcohol, methyl isobutyl ketone, ethyleneglycol monoethyl ether acetate, propyleneglycol monomethyl ether acetate and the like. Though excellent in the photosensitivity, the positive-working photoresist composition prepared by compounding this novolac resin is not fully practicable because the patterned resist layer formed therefrom is not highly resistant against heat and not free from occurrence of scum.
It is known that the heat resistance of the patterned resist layer formed from a positive-working photoresist composition can be improved by fractionating the novolac resin as the principal ingredient thereof to be freed from low molecular weight species but the fractionating treatment of the novolac resin requires an additional step in the preparation of the photoresist composition and is unfavorable against stabilization of the preparation process of the photoresist compositions.